Primary step in action of hormones on cells in binding of hormone to receptors specific for the hormone and located in or on cellular structure. Presence of estrogen receptors is accepted as important in predicting therapeutic benefit from estrogen therapy. Work now suggests that additionally assaying for progesterone receptor significantly improves predictive value of receptor testing. Recent information suggests that presence of both receptors is an indicator of clinical behavior of human breast cancer independent of other known prognostic factors, with survival advantage conferred on patients possessing both receptors on their tumor cells. Receptor testing is less widely done currently than these facts would suggest. Current tests for receptors are expensive, time- consuming and skill dependent. The most widely used techniques do not permit determination of whether receptors detected came from tumor or benign tissue cells - as procedures require homogenization of sample. Commercial tests for estrogen receptor are new to market; none currently exist for progesterone receptor. Phase I will develop fluorescent progesterone probe, and test it in animal and human clinical samples. Phases II will combine it with an existing company-developed estradiol probe. Company will then have most convenient, most complete system for assaying intact tissue for hormone receptors.